Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income		$ 167,365	$ 157,916	$ 144,786
Securities available for sale:
Net unrealized holding gain (loss) arising during the period		59,550	(32,816)	(17,848)
Reclassification adjustment for net loss (gain) realized in net income		178	(6)	(75)
Income tax (expense) benefit		(14,722)	9,244	6,811
Net change in unrealized gain (loss) on securities available for sale		45,006	(23,578)	(11,112)
Defined benefit pension plan obligation ¹:
Net unrealized holding gain (loss) arising during the period	[1]	0	(329)	329
Income tax (expense) benefit	[1]	0	125	(125)
Net change in unrealized gain (loss) in defined benefit pension plan obligation	[1]	0	(204)	204
Other comprehensive income (loss), net of tax	[1]	45,006	(23,782)	(10,908)
Comprehensive income		$ 212,371	$ 134,134	$ 133,878

[1] The Company's Board of Directors voted to terminate the defined benefit pension plan ("Pension Plan") effective February 1, 2018. Transfer of all Pension Plan assets, liabilities and administrative responsibilities were completed as of September 30, 2018.